ابر ماژلانی بزرگ (LMC) کهکشانی در همسایگی کهکشان راه شیری است. فاصلهاش از کهکشان راه شیری کمی کمتر از ۵۰ کیلو پارسک است و بنابراین سومین کهکشان نزدیک به راه شیری شمرده میشود. ابر ماژلانی بزرگ چهارمین کهکشان بزرگ گروه محلی است.
دریانوردان و سیاحان قدیم عرب که از دیرباز به سواحل خاوری آفریقا در آمد و شد بودند، نخستین کسانی بودند که با ابر ماژلانی بزرگ و ابر ماژلانی کوچک آشنا شدند.
نخستین اشارهٔ مستند به ابر ماژلانی بزرگ توسط اخترشناس ایرانی، عبدالرحمان صوفی در کتاب صورالکواکب در حوالی سال ۹۶۴ میلادی انجام شدهاست. عبدالرحمان صوفی نیز در واقع در کتاب صورالکواکب رصد آنها را گزارش میکند، نه رصد خود را. در کتاب صورالکواکب عبدالرحمان، با ترجمهٔ خواجه نصیر طوسی، دربارهٔ ابرهای ماژلانی چنین نوشته شدهاست:
… و قومی گفتهاند که در زیر سهیل دو کوکب است که قدمهای سهل اند و در زیر قدمهای سهل کواکب نورانی سپید است که به عراق نبینند و نه [بر متن/به زمین] نجد از ولایت عرب و اهل تهامه که بینند بَقَر خوانند آن کواکب را. و بطلمیوس از این هیچ نگفته و ما ندانیم که این سخن بر حق است یا باطل. و قومی گفتهاند ابتدای کواکب سفینه از نزدیک سعد بهام است …
ثبت رصد بعدی در سالهای ۱۵۰۳-۱۵۰۴ توسط آمریگو وسپوچی و در سومین سفر دریایی او انجام شد.
The next recorded observation was in 1503–4 by Amerigo Vespucci in a letter about his third voyage. In this letter he mentions "three Canopes [sic], two bright and one obscure"; "bright" refers to the two Magellanic Clouds, and "obscure" refers to the Coalsack.
Ferdinand Magellan sighted the LMC on his voyage in 1519, and his writings brought the LMC into common Western knowledge. The galaxy now bears his name.
The Large Magellanic Cloud has a prominent central bar and a spiral arm. The central bar seems to be warped so that the east and west ends are nearer the Milky Way than the middle. In 2014, measurements from the Hubble Space Telescope made it possible to determine that the LMC has a rotation period of 250 million years.
The LMC was long considered to be a planar galaxy that could be assumed to lie at a single distance from the Solar System. However, in 1986, Caldwell and Coulson found that field Cepheid variables in the northeast portion of the LMC lie closer to the Milky Way than Cepheids in the southwest portion. More recently, this inclined geometry for field stars in the LMC has been confirmed via observations of Cepheids, core helium-burning red clump stars and the tip of the red giant branch. All three of these papers find an inclination of ~35°, where a face-on galaxy has an inclination of 0°. Further work on the structure of the LMC using the kinematics of carbon stars showed that the LMC's disk is both thick and flared. Regarding the distribution of star clusters in the LMC, Schommer et al. measured velocities for ~80 clusters and found that the LMC's cluster system has kinematics consistent with the clusters moving in a disk-like distribution. These results were confirmed by Grocholski et al., who calculated distances to a number of clusters and showed that the LMC's cluster system is in fact distributed in the same plane as the field stars.
Location of the Large Magellanic Cloud with respect to the Milky Way and other satellite galaxies
The distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular. Cepheids have been shown to have a relationship between their absolute luminosity and the period over which their brightness varies. However, Cepheids appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period–luminosity relations. Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period–luminosity relation are more metal rich than those found in the LMC.
In 2006, the Cepheid absolute luminosity was re-calibrated using Cepheid variables in the galaxy Messier 106 that cover a range of metallicities. Using this improved calibration, they find an absolute distance modulus of 18.41, or 48 kpc (~157,000 light-years). This distance has been confirmed by other authors.
By cross-correlating different measurement methods, one can bound the distance; the residual errors are now less than the estimated size parameters of the LMC.
The results of a study using late-type eclipsing binaries to determine the distance more accurately was published in the scientific journal Nature in March 2013. A distance of 49.97 kpc (163,000 light-years) with an accuracy of 2.2% was obtained.
Two very different glowing gas clouds in the Large Magellanic Cloud
There is a bridge of gas connecting the Small Magellanic Cloud (SMC) with the LMC, which is evidence of tidal interaction between the galaxies. The Magellanic Clouds have a common envelope of neutral hydrogen indicating they have been gravitationally bound for a long time. This bridge of gas is a star-forming site.
No X-rays above background were observed from the Magellanic Clouds during the September 20, 1966, Nike-Tomahawk flight. A second Nike-Tomahawk rocket was launched from Johnston Atoll on September 22, 1966, at 17:13 UTC and reached an apogee of 160 km (99 mi), with spin-stabilization at 5.6 rps. The LMC was not detected in the X-ray range 8–80 keV.
Another Nike-Tomahawk was launched from Johnston Atoll at 11:32 UTC on October 29, 1968, to scan the LMC for X-rays. The first discrete X-ray source in Dorado was at RA 05h 20mDec −69°, and it was the Large Magellanic Cloud. This X-ray source extended over about 12° and is consistent with the Cloud. Its emission rate between 1.5–10.5 keV for a distance of 50 kpc is 4 x 1038 ergs/s. An X-ray astronomy instrument was carried aboard a Thor missile launched from Johnston Atoll on September 24, 1970, at 12:54 UTC and altitudes above 300 km (186 mi), to search for the Small Magellanic Cloud and to extend previous observations of the LMC. The source in the LMC appeared extended and contained the star ε Dor. The X-ray luminosity (Lx) over the range 1.5–12 keV was 6 × 1031 W (6 × 1038 erg/s).
DEM L316A is located some 160,000 light-years away in the Large Magellanic Cloud
The Large Magellanic Cloud (LMC) appears in the constellations Mensa and Dorado. LMC X-1 (the first X-ray source in the LMC) is at RA 05h 40m 05sDec −69° 45′ 51″, and is a high mass X-ray binary source (HMXB). Of the first five luminous LMC X-ray binaries: LMC X-1, X-2, X-3, X-4, and A 0538–66 (detected by Ariel 5 at A 0538–66); LMC X-2 is the only one that is a bright low-mass X-ray binary system (LMXB) in the LMC.
DEM L316 in the Large Magellanic Cloud consists of two supernova remnants.Chandra X-ray spectra show that the hot gas shell on the upper left contains a high abundance of iron. This implies that the upper-left SNR is the product of a Type Ia supernova. The much lower iron abundance in the lower SNR indicates a Type II supernova.
A 16 ms X-ray pulsar is associated with SNR 0538-69.1. SNR 0540-697 was resolved using ROSAT.
^ abMajaess, Daniel J.; Turner, David G.; Lane, David J.; Henden, Arne; Krajci, Tom (2010). "Anchoring the Universal Distance Scale via a Wesenheit Template". Journal of the American Association of Variable Star Observers. arXiv:1007.2300. Bibcode:2011JAVSO..39..122M.
^Peterson, Barbara Ryden, Bradley M. (2009). Foundations of astrophysics. New York: Pearson Addison-Wesley. p. 471. ISBN9780321595584.
^Mottini, M.; Romaniello, M.; Primas, F.; Bono, G.; Groenewegen, M. A. T.; François, P. (2006). "The chemical composition of Cepheids in the Milky Way and the Magellanic Clouds". MmSAI. 77: 156. arXiv:astro-ph/0510514. Bibcode:2006MmSAI..77..156M.
^ abcMark, Hans; Price, R; Rodrigues, R; Seward, F. D; Swift, C. D (Mar 1969). "Detection of X-rays from the large magellanic cloud". Astrophysical Journal Letters. 155 (3): L143–4. Bibcode:1969ApJ...155L.143M. doi:10.1086/180322.